1. Field of the Invention
The present invention relates generally to an apparatus and method for coding/decoding in a mobile communication system using a Multiple Input Multiple Output (MIMO) scheme (hereinafter referred to as a “MIMO mobile communication system”), and in particular, to an apparatus and method for coding/decoding pseudo orthogonal space-time block codes that maximize a coding gain and a data rate.
2. Description of the Related Art
In communication systems, the most fundamental issue is how efficiently and reliably the communication system can transmit data over a channel. As the next generation multimedia mobile communication system, into which extensive research is being conducted, requires a high-speed communication system capable of processing and transmitting a variety of information such as image and radio data surpassing the early voice-oriented services, it is essential to increase system efficiency using a channel coding scheme suitable for the system.
However, the wireless channel environment prepared in the mobile communication system, unlike the wire channel environment, is subject to inevitable errors due to several causes such as multipath interference, shadowing, propagation attenuation, time-varying noise, interference, fading, etc., each causing a loss of information.
The information loss distorts the actual transmission signals, causing deterioration in the overall performance of the mobile communication system. Generally, in order to reduce the information loss, various error control techniques are used to increase system reliability. The most basic one of the error control techniques is a technique using error correcting codes.
Further, in order to prevent the instability of communication due to fading, diversity schemes are used, and the diversity schemes are roughly classified into a time diversity scheme, a frequency diversity scheme, and an antenna diversity scheme, i.e. a space diversity scheme.
The antenna diversity scheme, which is a scheme using multiple antennas, is classified into a receive antenna diversity scheme using a plurality of reception antennas, a transmit antenna diversity scheme using a plurality of transmission antennas, and a MIMO scheme using a plurality of reception antennas and a plurality of transmission antennas.
The MIMO scheme is a Space-Time Coding (STC) scheme, and the space-time coding scheme extends a time-domain coding scheme to a space-domain coding scheme by transmitting signals coded with a coding scheme using a plurality of transmission antennas, thereby achieving a lower error rate. The space-time coding scheme has been developed on the assumption that a radio channel between a transmitter and a receiver is a flat fading channel.
As a Space-Time Block Coding (STBC) scheme, which is one of the schemes that utilizes the antenna diversity scheme, i.e. a transmit antenna diversity scheme or a space-time block coding scheme, proposed by S. M. Alamouti (cf. S. M. Alamouti, ‘A simple transmitter diversity scheme for wireless communications’, IEEE Journal on Selected Area in Communications, Vol. 16, pp. 1451-1458, October 1998), and a space-time block coding scheme proposed by Vahid Tarokh (cf. Vahid Tarokh, ‘Space-time block coding from orthogonal designs’, IEEE Trans. on Info., Theory, Vol. 45, pp. 1456-1467, July 1999). For convenience, hereinafter, the space-time block coding scheme proposed by S. M. Alamouti will be referred to as an “Alamouti space-time block coding scheme” and the space-time block coding scheme proposed by Tarokh will be referred to as a “Tarokh space-time block coding scheme.”
The Alamouti space-time block coding scheme is a space-time block coding scheme in which a transmitter uses 2 transmission antennas, and the Tarokh space-time block coding scheme indicates a space-time block coding scheme modified by extending the Alamouti space-time block coding scheme such that it can be applied to a transmitter that uses more than 2 transmission antennas. Because the performance gain acquired when the space-time block coding scheme is combined with the MIMO scheme is well known, a description thereof will be omitted herein.
Active research into a 4th generation (4G) communication system which is the next generation communication system is being conducted to provide to users services with various Quality-of-Service (QoS) levels at a high data rate. Currently, in the 4G communication system, research is being conducted on technology for supporting high-speed services in such a way that mobility and QoS are guaranteed in a Broadband Wireless Access (BWA) communication system such as a wireless Local Area Network (LAN) system and a wireless Metropolitan Area Network (MAN) system.
In the 4G communication system, active research is being conducted in an Orthogonal Frequency Division Multiplexing (OFDM) scheme as a scheme useful for high-speed data transmission in wire/wireless channels. The OFDM scheme, a scheme for transmitting data using multiple carriers, is a type of a Multi Carrier Modulation (MCM) scheme that converts a serial input symbol stream into parallel symbols and modulates the parallel symbols with multiple orthogonal subcarriers before transmission.
The 4G communication system needs broadband spectrum resources in order to provide high-speed, high-quality wireless multimedia services. However, when the broadband spectrum resources are used, a fading effect in a radio transmission path due to multipath propagation is considerable, and a frequency selective fading effect occurs even in a transmission band. Therefore, for the high-speed wireless multimedia services, the OFDM scheme, which is robust against the frequency selective fading, is higher in gain than a Code Division Multiple Access (CDMA) scheme. Thus, the 4G communication system has a tendency to chiefly use the OFDM scheme.
As described above, the wireless channel environment provided in the mobile communication system, unlike the wire channel environment, experiences information losses caused by inter-symbol interference (ISI) and frequency selective fading due to multipath interference, shadowing, propagation attenuation, time-varying noise, delay spread, etc. Unavoidable errors occur due to the information losses, causing a loss of information data. The loss of information data considerably distorts the actual transmission signals, causing deterioration in the overall performance of the mobile communication system.
A mobile communication system using the OFDM scheme (hereinafter referred to as an “OFDM mobile communication system”) not only uses the multiple orthogonal subcarriers but also inserts guard intervals in transmitting the OFDM symbols in order to cope with the inter-symbol interference and the frequency selective fading. The guard intervals are inserted using one of the following two schemes: a cyclic prefix scheme of copying a predetermined number of last samples of a time-domain OFDM symbol and inserting the copied samples into an effective OFDM symbol, and a cyclic postfix scheme of copying a predetermined number of first samples of a time-domain OFDM symbol and inserting the copied samples into an effective OFDM symbol. As a result, the OFDM mobile communication system uniformly distributes the frequency selective fading effect to the multiple subcarriers so that a receiver can receive signals even though it has a single-tap equalizer for each of the subcarriers.
The space-time coding scheme faces a tradeoff between the following 3 aims of (1) maximizing error performance, i.e. maximizing a diversity order and a coding gain, (2) maximizing a coding rate, and (3) minimizing decoding complexity. That is, there is no space-time coding scheme that satisfies all of the 3 aims, and when one of the 3 aims is satisfied, other aim(s) is/are sacrificed.
For example, in the flat fading channel, it is most important to optimize the diversity order using the space-time coding scheme. On the contrary, in a frequency selective fading channel for which the frequency diversity is useful, it is less important to maximize the diversity order, i.e. acquire the full diversity gain, using the space-time coding scheme. This is because in the frequency selective fading channel, it is possible to maximize a diversity gain using the OFDM scheme as described above.
Accordingly, there is a demand for a new space-time block coding scheme for maximizing a data rate and a coding gain while securing a full diversity gain in the MIMO mobile communication system.